A workstation-based NWP scheme is described which allows forecasters to reposition and change in intensity features such as fronts and depressions, and thus alter model fields in a realistic way. The technique is based on distortion of either geopotential height or potential vorticity (PV). In the latter case, the PV is calculated from geopotential distribution, distorted, then inverted to yield a modified, yet consistent and dynamically balanced, set of grid-point data.